1. Field of the Invention
The present invention is generally directed to9 a photoelectric measuring system, and more particularly, an apparatus and method for coupling light beams in and out of an integrated optical circuit.
2. Description of the Prior Art
A wide variety of photoelectric measuring devices exist, however, they all suffer from disadvantages the present invention eliminates. Optical displacement measuring systems which are generally known in the art use a diffraction grating which consists of very thin grid lines placed close together on a metal or glass plate.
Typically, available optical displacement measuring devices also comprise a light source which emits monochromatic light, for example a laser light source, and a detector. The light beam from the light source is diffracted by the grating and the two diffracted beams are caused to interfere with each other with the use of a mirror arrangement. The interfered light generates periodically varying measurement signals which are detected by a detector. Such devices are described, for example, in German patent specification DE-OS 33 16 144 and Japanese unexamined patent specification JP-OS 59 164 914.
This type of arrangement suffers from a major disadvantage, namely it requires a trade off between accuracy and the space required by the measuring device. If the device is used in a relatively small space, the relative position of the optical system relative to the diffraction grating is shifted in the direction of the grid lines of the diffraction grating. Thus, a phase change may take place which is similar to the phase change which occurs when relative movement occurs perpendicular to the plane of the diffraction grating. Obviously, this decreases the accuracy of the measurement system.
Alternatively, if the light beam is arranged such that it enters in a substantially vertical manner, the accuracy problem described above is reduced but the space required for the system is increased.
German Patent Application P 36 25 327.8-82 (corresponding to U.S. Pat. application Ser. No. 07/077,190 now U.S. Pat. NO. 4,938,595) describes a position measuring system which is simple in construction and yet avoids undesirable environmental influences on the measuring device so that dependable operation is provided. FIG. 1 illustrates this prior art device. A transmitted light measurement device uses a light source (not shown) and a moveable diffraction grating 2 perpendicularly arranged to the direction of emission from the light source. The light source may be, for example, mounted in the base of a machine tool (not shown) and the diffraction grating 2 may then be, for example, similarly mounted in the cradle of the machine tool (not shown). The relative movement between the base and cradle is measured as machine movement. Thus, this relative movement corresponds to the relative displacement between the light source and the diffraction grating 2.
The beam of the light source is diffracted on the diffraction grating 2 and partial beam bundles, 1' and 1" are created. The partial beam bundles, 1' and 1", fall upon an integrated optical circuit 5. Optical circuit 5 is contained on a substrate in a manner familiar to those who practice in the art. Two coupling means 3 and 4, two beam waveguides (not shown), a coupler (not shown), as well as three detectors (not shown) are components of the integrated optical circuit 5. The coupling-in grids 3 and 4 are in the broadest sense parabolic. While the diffraction grating 2 can be realized with a grid constant between 1.5 .mu. to 20 .mu., coupling-in grids 3 and 4 have a grid constant on the order of 0.5 .mu.. Parabolic coupling-in grids having a grid constant of that order are difficult and expensive to construct and produce.
Accordingly, it is a primary object of the present invention to provide a measuring device which utilizes a simple grating structure thus facilitating the construction of the device.
Another object of the present invention is to provide a measuring device that may be economically produced and yet provides a high degree of precision in a small structure.
Further objects and advantages will become apparent from the following description and the accompanying drawings.